1. Field of the Invention
This invention pertains to mechanisms for filtering dust or particulate matter from air streams. It discloses a method, heretofore unknown, for backflushing the filtration elements ordinarily used to separate the dust from dust-laden air.
2. Description of Prior Art
The conventional dust collector, those operating by filtration, consists of a cabinet essentially airtight and operating throughout its internal parts at sub-atmospheric pressure created by a blower at the point where cleaned air is discharged from the cabinet to free air. Thus, any inadverdant leakage being inward, dust cannot be spread to the vicinity. The inlet to the cabinet directs dirty air into a dirty air compartment inside the cabinet, which also has a clean air compartment. Filters hang in the dirty air compartment with their interior ported to the clean air compartment, thus collecting dust particles on the exterior surface of the filtration element. After a short period of operation, the tiny passages for air through the filtration element become even smaller due to the buildup of dust thus effected. Eventually, these passages of the filtration element becomes so clogged they pass too little volume of air and that with too much pressure drop, and the filtration element needs cleaning of the collected dust.
Cleaning has been conventionally done in several ways, beating and a short burst of air in a high pressure backflow being the usual methods. Both agitate the wall portion of filter, say a cartridge carrying porous thick paper-like material heavily pleated so as to expose an expanded area to the dust-laden air without taking up too much cabinet space. The backflow is preferred today because it dislodges the accumulated dirt without damaging the filtration element material; physical agitation can damage it. But with high pressure air bursts, a certain amount of physical agitation is inevitable. Both conventional methods have another drawback, however.
The designer of the system knows that it must be sized to operate ideally with a basic amount of passage-clogging due to the buildup of exterior dust. So the original pore size of the filtration element is oversize, and the designer counts on a certain layer of dust called the "cake" being present even after cleaning. This is a designed-in property of the overall system; thoroughly cleaned filtration elements would pass too large particles with too low pressure drop. For reference, as the dust layer builds up in thickness, it filters ever finer and the pressure drop through it continually rises. The designed operating dust-layer thickness range is not from zero to some selected thickness, but from a cake thickness to some selected thickness. If the cake is not present locally, the filtration is locally such that over-large particles pass through. So physical agitation may not only damage the structure of the filtration element, but may also cause a local and temporary condition of improper, inadequate performance.
My invention, depending as it does on a relatively gentle backflow with minimum deflection of, say the cartridge walls (thick paper-like material), avoids the above problems with conventional cleaning methods as fully as they can be avoided in practice.
In the prior art one other device (Bourne, U.S. Pat. No. 3,648,442) incorporates the advantages described above. Bourne obtains clean air from the clean-air plenum or compartment, passes it through a blower located inside or outside the cabinet, and directs it into the filter discharge passages using an arm which sweeps circularly around the clean-air compartment. My invention dispenses with his blower and arm and the associated mechanism, replacing them with a much cheaper and more reliable valve, one for each cartridge.
In the prior art no provision was generally made for preventing the dislodged dust from the filter being cleaned from passing directly to adjacent filters which are normally still operating during the cleaning process. My observations during tests I have conducted show this is exactly what occurs. Remillieux (U.S. Pat. No. 4,292,053) teaches using partial separation walls amid the hanging filters to reduce this effect. In my preferred dust collector I have provided dust shields surrounding the cartridge filters and installed with them, avoiding the problem with his concept of getting rid of the shields or working between them during filter replacement. This save down-time.